This invention relates in general to loudspeakers which produce sound in response to an audio signal, and more particularly to a loudspeaker with an improved air cooling system.
Conventional loudspeakers typically include a cone-shaped diaphragm which is vibrated by an electromechanical driver. The driver generally comprises a magnetic structure and a voice coil located within a gap of the magnetic structure. The voice coil in turn is rigidly attached to the diaphragm. Alternating voltage in the audio frequency range is applied to the terminals of the voice coil causing a corresponding alternating current to flow through the voice coil. The interaction between the current flowing through the voice coil and the magnetic field present in the gap of the magnetic structure causes the voice coil to move either toward or away from the magnetic structure. Since the voice coil is rigidly attached to the diaphragm of the loudspeaker, the movement of the voice coil drives the diaphragm, thus producing acoustical output from the loudspeaker.
A substantial portion of the impedance associated with electromechanical drivers is caused by the wire that forms the voice coil due to the wire's DC resistance. Accordingly, most of the electrical power applied to the voice coil is converted into heat rather than sound. The ultimate power handling capacity of the voice coil, and thus the loudspeaker, is limited by the ability of the device to tolerate heat. Heat tolerance is generally determined by the lowest melting point of wire insulation and other components, as well as the heat capacity of the adhesive used to construct the voice coil.
The problems produced by heat generation are further compounded by temperature-induced resistance, commonly referred to as power compression. As the temperature of the voice coil increases, the DC resistance of the copper or aluminum conductors or wires used in the voice coil also increases, resulting in progressively decreasing efficiency. For example, a copper wire voice coil that has a DC resistance of approximately eight ohms at 68° C. will have a DC resistance of approximately 16 ohms at 270° C. At 270° C., the voice coil will draw less power from the voltage applied to its terminals, and a substantial portion of the power that it does draw will be converted into heat. Consequently, the loudspeaker, which is a relatively inefficient transducer at room temperature, will be further reduced in efficiency at high voice coil temperatures. This power compression increases as the voltage applied to the voice coil increases, and can reach a point where a further increase in applied voltage results in virtually no increase in acoustical output, only a further increase in heat.
In an attempt to reduce the problems associated with voice coil heating, U.S. Pat. No. 4,757,547 issued to Danley on Jul. 12, 1988, discloses cooling a voice coil by blowing air between the voice coil windings and the boundaries of the magnetic gap. Typically, the clearances between the voice coil and the boundaries of the magnetic gap are quite small, usually under 0.020 inch. Forcing sufficient air through these clearances to significantly cool the voice coil requires relatively high air flow at relatively high pressure through the small clearances surrounding the voice coil, resulting in undesirable noise and distortion in the loudspeaker. This patent also discloses connecting the blower in parallel with the audio signal source, either directly or through a rectifier. Connection of the blower in this manner can potentially cause excessive current to be drawn from the audio signal source at high operating levels, possibly exceeding the power capacity of the audio amplifier.
U.S. Pat. No. 4,811,403 issued to Henricksen et al. on Mar. 7, 1989, discloses a cooling system with a thermally conductive load bearing member and a plurality of loudspeakers in thermal engagement with the load bearing member. Air flow, which may be by forced air circulation, cools the load bearing member to thereby cool the loudspeaker that is in thermal engagement with the load bearing member. This cooling method requires a special enclosure of complex design in order to function properly. In addition, the loudspeaker is not in direct contact with the cooling air flow.
U.S. Pat. No. 5,426,707 issued to Wijnker on Jun. 20, 1995, discloses cooling a loudspeaker by forcing air through the narrow gaps between the voice coil and the boundaries of the magnet gap. This is similar to the method disclosed in U.S. Pat. No. 4,757,547, and also would result in undesirable noise and distortion in the loudspeaker.
It is therefore desirable to provide a loudspeaker system that can be cooled during operation without drawing excessive current from the audio signal source. It is further desirable to provide a loudspeaker system with forced air circulation for expelling heat generated by the loudspeaker driver mechanism out of the loudspeaker enclosure to thereby increase both the efficiency and power capacity of the loudspeaker, as well as its reliability and service life.